Numerous ion-selective sensors designed for monitoring of hydrogen ions in various process fluids are widely known.
Such examples include, in particular, a ion-selective sensor described by P. Meier, A. Lohrum and J. Gareess (Editors) in the study, "Practice and Theory of pH Measurement", Ingold Messtechnik A G, CH-B902 Undorf/Switzerland, 1989, p. 15. This combined sensor comprises a glass tube body incorporating glass indicator and reference electrodes filled with a liquid electrolyte.
Another known ion-selective sensor (a Switzerland's Ingold booklet "pH/Redox Measurement in Biotechnology. pH-Electrodes. Industrial Probes. Sensors," Ingold Messtechnik A G, CH-8902 Urdorf/Switzerland 1990, p. 10) has a similar design, but its reference electrode is filled with a gel-type electrolyte.
The both above mentioned sensors have a monolithic glass design and require refillings of their reference electrodes. These sensors are only suitable for predefined spatial positions and are inoperable in harsh conditions (elevated temperatures and those below 0.degree. C., high pressures, zero-gravity and other conditions). Sensors intended for redox potential monitoring are widely employed at present. For instance, a sensor with a reference electrode filled with gel electrolyte and an indicator electrode with a platinum or gold sensing element have been described in a booklet titled "PH/Redox Measurement in Chemical Processes. pH-Electrodes. Industrial Probes. Sensors," published by Ingold Messtechnik A G, CH-8902 Urdorf/Switzerland, 1990, p. 16.
Similarly to the above mentioned ion-selective sensors, this device comprises an integral, monolithic glass design which is only operational in a predefined spatial position and is not intended for harsh conditions.
Another known ion-selective sensor (the US' Foxboro Company booklet, "pH/ORP/ISE Sensors B71 pH Series," The Foxboro Company, Foxboro Mass. 02035-2099, 1993, p. 1) was designed for fluorine and hydrogen ion activity monitoring and is suitable for redox potential monitoring applications.
This sensor incorporates a plastic housing tube where a detachable indicator electrode is rigidly fixed in addition to a reference electrode that has a hollow body filled with liquid or gel electrolyte containing a solid dispersed material which forms a spatial structure that is freely movable inside the reference electrode. An indicator electrode filled with liquid electrolyte, together with a solid ion-selective sensor, are installed into the housing tube for ion activity monitoring, and an indicator electrode with a platinum sensing element is inserted into the housing tube for redox potential monitoring.
Compared to the previously mentioned sensors, the claimed device is more flexible in use and therefore has wider applications.
However, all above mentioned disadvantages apply to the latter sensor. They are attributed to the liquid or gel-type electrolyte present in the reference electrode and the liquid electrolyte inside the indicator electrode. This requires frequent refillings of the reference electrode electrolyte, poses spatial limitations for sensor operation and restricts harsh-environment applications.